Conventional actuators have a moveably supported member, and a coil. When a current is passed through the coil, the member is urged to move. A control circuit is coupled to the coil in order to selectively supply current to the coil. One example of such an arrangement is a hard disk drive, where the moveable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement relative thereto. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. The movement of the member to one end of its path of travel is commonly referred to as a retract of the member, and in the context of a hard disk drive corresponds to parking of the read/write head on the member.
When a current is applied to the coil of the actuator, the member is urged to accelerate at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-emf voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. While known techniques of measuring the back-emf voltage have been adequate for their intended purposes, they have not been satisfactorily in all respects.
In particular, because a current is being applied to the coil in order to urge movement of the member, and because the actuator has internal resistance, the current flow and resistance together generate voltages which are additional to the back-emf voltage, and which thus obscure accurate measurement of the back-emf voltage. This is complicated by the fact that the internal resistance of the actuator can vary dynamically, due to temperature changes and other factors. One known approach measures the voltage and current applied to the actuator, and calculates the back-emf voltage Vbe as follows, where Rm is the internal resistance of the actuator: EQU Vbe=(I.sub.applied.multidot.Rm)+(Ldi/dt)-V.sub.applied.
(where Ldi/dt is assumed to be approximately=0).
Since the actual value of the actuator resistance Rm will change dynamically, the accuracy of this known technique will fluctuate unpredictably in dependence on whether, at the time of a particular calculation, the actual value of Rm is close to or different from the predetermined constant value used for Rm in the calculation.
A related consideration is that known retract methods control the magnitude of voltage applied across the coil, which is usually adequate for controlling the rate at which the moveable member of the actuator retracts, because the retract rate is proportional to the applied voltage minus the load losses. In some systems, however, the load can change dramatically, which will translate into undesired changes in rate.
A further disadvantage of the foregoing calculation is that a microprocessor or other circuitry needed to solve the equation may be prohibitly complex and expensive, particularly where it is desirable to implement the entire system in a single integrated circuit. Also, in a situation where the power is failing, it is typically impractical to try to maintain sufficient power to a microprocessor to permit it to continue to operate. A further disadvantage is that, when the desired velocity of the moveable member is small, as is the case in most retract scenarios, small changes to the actuator resistance Rm can lead to large errors in the measurement of back-emf voltage using traditional techniques. In extreme cases, this can lead to positive feedback within the control loop, which of course is undesirable.